The microstructure and creep properties of submicrocrystalline nickel and its nanocomposite processed by electrodeposition were evaluated. The results show that the creep resistance of the nickel nanocomposite reinforced by nano-sized SiO2 particles at temperatures in the range from 293 to 573K may be improved in comparison with the unreinforced nickel.

In this work, we present and explain evidence of a charge confinement phenomena exhibited by SnO2 nanoparticles. By applying a voltage pulse from the scanning tunneling microscope (STM) tip or momentarily increasing the gap voltage, charged features can be written on the surface. The voltage dependence of the apparent height of the features, and current imaging tunneling spectroscopy experiments, supports the charge confinement theory put forward. This is substantiated by a numerical model illustrating how charge is confined in individual nanoparticles, and is used to explain the observations of STM experiments.

Abstract Magnesium micropowder was mixed and ball-milled with 3 vol.% graphite powder. The Mg + Graphite composite was deformed in compression at temperatures between room temperature and 300 °C at a constant crosshead speed, giving an initial strain rate between 1.4 · 10 –5 and 1.4 · 10– 3 s –1. The yield stress of the composite is higher than that of unreinforced magnesium prepared with the same technology. The yield stress and the maximum stress decrease rapidly with increasing temperature. At temperatures higher than room temperature, the true stress– true strain curves exhibit a work-hardening rate close to zero, which indicates a dynamic equilibrium between hardening and softening. Double cross-slip of screw components of c + a dislocations is considered as the main recovery mechanism.

Internal friction in ultra-fine grained Mg with 3vol% of Graphite was measured by forced vibration method at low frequencies of 0.1, 0.5, 1.0, and 2.0 Hz over a temperature range from room temperature to 753 K with continuous heating. The specimens were prepared by milling procedure in an inert atmosphere and subsequent compacted and hot extruded. Two developed peaks in the internal friction spectrum were obtained at temperatures ≈ 350 K and ≈ 550 K. While the position of the first peak is frequency dependent, the second peak position is stable, independent of measuring frequency. The activation energy of the low temperature peak was estimated. In the light of internal friction measurements, the high temperature internal friction peak is attributed to the generation and motion of dislocations produced by the difference in the coefficient of thermal expansion between the Mg matrix and Gr phase at the matrix–particle interfaces.

A method was developed which allows grain-refining of Mg alloys via casting from Mg melts containing nanoscale TiN particles. AZ91 and AZ31 precursor materials loaded with 10 to 13 vol.-% of TiN nano particles was generated and added to pure AZ91 and AZ31 melts, respectively. The dispersed particles in the melt act as nucleation sites of Mg grains during solidification. The cast materials containing TiN show refined grains and less and smaller dendrites in comparison to the pure Mg alloys cast under the same metallurgical conditions. The grain refined materials contain about 0.6 vol.-% of TiN nano particles. The precursor materials were analyzed by X-ray diffraction and differential thermo analysis. It was shown that TiN nano particles are stable on AZ91 melts for several hours at 700 degrees C. This is a requirement to gain a heterogeneous nucleation of Mg grains on TiN seeds. The results are discussed.

A comparison between the tensile properties and the creep characteristics of the electrodeposited unreinforced nickel and its nanocomposite reinforced by 2 vol.% of nano-sized SiO(2) particles at temperatures in the range from 293 to 473K shows that the tensile properties of the composite are not considerably improved compared to those of the matrix nickel. By contrast, the presence of the particle reinforcement may lead to an increase of the creep resistance of the composite.

The dispersion behavior of Al2O3 and SiO2 nanoparticles in nickel sulfamate plating baths of different compositions were investigated by Zeta potential measurements and photon correlation spectroscopy (PCS). The Al2O3 particles have a median diameter of 25 nm and are crystallized in the delta- and g-phase. The amorphous SiO2 particles have a median diameter of 7 nm. In the case of dispersions with Al2O3 particles it was found by PCS that their agglomeration tendency increases with increasing ion concentration of the bath and therefore obeys qualitatively the Delaguin-Landau-Verwey-Overbeek (DLVO) theory. SiO2 particles in the same electrolytes show a different behavior: At a specific ion concentration the point of zero charge (isoelectric point) is shifted to higher pH values. At the highest ion concentration investigated, the particle charge measurements returned zeta potential values of about 115 mV and higher even at pH values up to 6. This result could be explained by an adsorption of Ni2+ ions on the SiO2 particle surface. PCS at this dispersion reveals low agglomeration of the SiO2 nanoparticles in the employed plating bath. The results are discussed. (c) 2005 The Electrochemical Society.

We have investigated the effect of O2 and reducing gases (CH4 and CO) exposure on nanocrystalline SnO2 in vacuum and at elevated temperatures (120°C) using three different techniques: X-ray photoelectron spectroscopy, in vacuum resistance measurements and scanning tunnelling microscopy and spectroscopy. XPS and resistance measurements showed that O2 chemisorbtion causes an upward surface band bending of 0.2eV and a resistance increase of 50MΩ while CH4 exposure resulted in a 0.1eV downward band bending and a 20MΩ drop in resistance. Stable STM imaging at 120°C was achieved and clearly resolved the 8nm particles. STS measurements indicate a change in the surface electronic properties of the SnO2 particles following O2 exposure.

In this work, we report on the ability to write features less than 15nm in size on nanocrystalline SnO2 by applying negative voltage pulses onto an STM tip. The fact that these features can be erased by scanning with a positive tip bias, and the strong dependence of the apparent height of the features with scanning bias after writing seems to indicate that a degree of charge confinement within the 8nm nano-crystals is involved.

The drive towards nanotechnology has highlighted the need to engineer the properties of surfaces in unprecedented detail. Here, we report the modification of nanocrystalline SnO2 surfaces using the tip of a scanning tunneling microscope (STM) to inject electrons into individual 8 nm SnO2 nanocrystals. The surface displays a characteristic consistent with charge retention within the grains producing dramatic enhancements in the effective height of the nanoparticles, as observed by STM imaging. This allows the production of modified surfaces where patterns can be written onto a surface with a spatial resolution limited by the size of tip and the nanoparticles, 8 nm in this case. It is also possible to selectively erase the features on the surface using the STM tip under reverse bias. The pattern remains for up to three weeks and therefore opens the door to applications such as patterned nanoscale catalysis, molecular docking, and even ultrahigh density analog data storage.

Mg was dispersion-strengthened with graphite by powder metallurgy. The material was produced by ball milling Mg micropowder (median particle diameter 40 mum) with 3 vol.-% graphite powder (median particle diameter 1-2 mum). After 8 h ball milling the product was consolidated by hot extrusion. Structural analysis revealed that a submicrocrystalline structure developed during ball milling. Tensile tests showed that the material was brittle even up to 300 degreesC and, therefore, most mechanical tests were carried out under compression. Under those conditions the reinforced material showed yield stresses of 270 MPa at ambient temperature, 170 MPa at 150 degreesC, and 125 MPa at 300 degreesC. Mg processed under the same conditions, but without graphite addition, had significantly lower yield stresses. The dispersion-strengthened Mg showed a marked increase in creep resistance: at 200 degreesC and a stress, sigma(c), of 100 MPa, the secondary creep rate, epsilon(s), was in the lower 10(-9) s(-1) range and at 300 degreesC and sigma(c) of 80 MPa, epsilon(s) values of up to 1 x 10(-8) s(-1) were measured. The results are discussed.

The effect of severe plastic deformation by cold high pressure torsion (HPT) on radio frequency (RF) plasma nitriding of pure iron, as well as St2K50 and X5CrNi1810 steels was investigated. Nitriding was carried out for 3 h in a nitrogen atmosphere at a pressure of 10−5 bar and temperatures of 350 and 400 °C. Nitrided specimens were analysed by scanning electron microscopy (SEM), X-ray diffraction and micro hardness measurements. It was found that HPT enhances the effect of nitriding leading almost to doubling of the thickness of the nitrided layer for pure iron and the high alloyed steel. The largest increase in hardness was observed when HPT was combined with RF plasma nitriding at 350 °C. In the case of pure iron, the X-ray diffraction spectra showed the formation of ε and γ′ nitrides in the compound layer, with a preferential formation of γ′ at the expense of the α-phase at the higher nitriding temperature. The corresponding surface hardness was up to 950 HV0.01. While the HPT-processed St2K50 exhibits both nitride phases after nitriding at 350 °C, only the γ′-phase was observed after nitriding at 400 °C. A surface hardness of up to 1050 HV0.01 was measured for this steel. The high alloyed steel X5CrNi1810 exhibited the highest increase in surface hardness when HPT was combined with nitriding at 350 °C. The surface hardness of this steel was greater than 1400 HV0.025. The XRD analyses indicate the formation of the expanded austenite (S-phase) in the surface layer as a result of RF plasma nitriding. Furthermore, after HPT X5CrNi1810 was transformed completely into deformation martensite which did not transform back to austenite under thermochemical treatment. However, in the case of nitriding of the HPT-processed high alloyed steel at 400 °C, the formation of the S-phase was less pronounced. In view of the observed XRD peak broadening, the formation of nitrides, such as e.g. CrN, cannot be ruled out.

Strain amplitude dependence of the logarithmic decrement was measured on microcrystalline and nanocrystalline magnesium unreinforced and reinforced with nanoparticles of Al2O3, ZrO2, and graphite. Measurements were carried out before and after step by step isochronal thermal treatment at increasing temperature. A decrease in the grain size and secondary phases increase the strain independent component of the decrement. Microstructure changes due to thermal treatment are responsible for an increase of the strain dependent component of the decrement.

The nitriding behaviour of specimens of severely deformed Armco iron and St2K50 steel was compared with that of as-received specimens of the same materials. Radio frequency (RF) nitriding with −1 kV bias was performed for 3 h at 350 and 400 °C in pure nitrogen atmosphere at a pressure of 10−5 bar. Results of scanning electron microscopy, glancing angle XRD, as well as hardness and corrosion measurements are presented and discussed with regard to the influence of severe deformation on the nitriding behaviour, particularly nitride formation and nitride layer growth, and on the mechanical and corrosion properties. The results show a remarkable increase of the nitrogen inward diffusion and enhanced corrosion resistance of severely deformed material.

The effect of severe plastic deformation by high pressure torsion (HPT) on radio frequency plasma nitriding of X5CrNi1810 steel was investigated. The results reveal that HPT enhances the effect of nitriding at 350 °C leading to doubling of the nitrided layer thickness. This layer was shown to consist of expanded austenite and possibly some iron nitrides.